Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this disclosure and are not admitted to be prior art by inclusion in this section.
A klystron is a type of high radio frequency (RF) amplifier (e.g., microwave amplifier), which can be used in power sources for electron accelerators and ultra high frequency (UHF) transmitters for radar, television, and satellite communication, as well as a drive power generator for particle accelerators. The klystron can be used in medicine, security and inspection, active denial, material processing, and high energy physics applications. The klystron is an electron device that includes a hollow tube structure (e.g., hollow metallic waveguide) that operates in a high vacuum (e.g., a vacuum device, vacuum electron device, or vacuum electric device). In a klystron, an electron beam generated by an electron gun interacts with radio waves as the electron beam passes through resonant cavities (e.g., metal box or cylindrical type shapes) along the length of a tube (e.g., a drift tube). The electron beam passes through a first cavity to which an input signal is applied. The energy of the electron beam amplifies the signal in the resonant cavities, and the amplified signal is taken from a later cavity at the other end of the klystron. In a conventional round beam klystron (or annular beam klystron [ABK]), a cylindrical shaped electron beam, confined by a magnet, traverses and interacts with a number of resonant cavities, amplifying an input signal often by 30-60 decibels (dB; i.e., gain of a thousand to a million times). The high RF fields generated by the cavities are isolated from other cavities by the cylindrical beam drift tube, which may be too small to propagate an RF field below a specified frequency, referred to as a cutoff frequency. The size of the drift tube, the electron gun, and focusing magnetic fields (e.g., B-fields) can place an upper limit on the current, and hence the power, of the klystron.
The sheet beam klystron (SBK) is a microwave power amplifier that can be a smaller or lower cost alternative to conventional round beam klystrons, can produce more average power than the round beam klystrons, and can extend to higher frequencies more readily than round beam klystrons. Due to the relatively wide and flat structure of the cavities and drift tube in the SBK, the SBK can be unstable. Electromagnetic (EM) radiation confined to hollow structures can have transverse modes, such as transverse electric (TE) modes, transverse magnetic (TM) modes, and hybrid modes. A transverse mode is a particular electromagnetic field pattern of radiation measured in a plane perpendicular (i.e., transverse) to the propagation direction of the beam of electromagnetic radiation. The TE mode (or H mode) is an electromagnetic field pattern without an electric field in the direction of propagation (i.e., a magnetic [H] field occurs along the direction of propagation). The TM mode (or E mode) is an electromagnetic field pattern without a magnetic field in the direction of propagation (i.e., an electric [E] field occurs along the direction of propagation). The hybrid mode is an electromagnetic field pattern with a non-zero electric field and a non-zero magnetic field in the direction of propagation. The resonant cavities amplify the RF field of the input while the resonant cavities in combination with drift tubes effect the gain and bandwidth of the klystron, which is often referred to as a tube. In the SBK, resonant cavities and drift tube may allow some transverse modes, referred to as trapped modes or parasitic modes, to be excited and grow.
Instabilities in a klystron can occur when positive feedback occurs between a transverse mode (or propagating mode) and an induced current on a quasi-steady state electron beam emitted by the electron gun (or electron beam generator). The wide drift tube of the SBK can support propagating modes, which can be “trapped” (i.e., form standing waves with strong transverse electric fields [e.g., TE mode] that can drive the electron beam into the drift tube walls), which can cause the electron beam to become unstable (e.g. TE mode instability). Instabilities in the klystron can result in the dampening of the RF fields of the signal or the electron beam colliding with the walls of the tube (e.g., a drift tube) of the SBK, as shown in FIG. 1, which can reduce the amplification of the RF signal, dampen the output signal, or damage the klystron. FIG. 1 illustrates a seven-cavity SBK 160 with a wave form of an electron beam 170 changing while passing through resonant cavities 164A-G in a drift tube 162, which results in the instability of the electron beam 172. Although the electron beam is shown hitting a drift tube wall between the sixth and seventh resonant cavities 164F-G, the instability of the electron beam is shown to occur as early as the second resonant cavity 164B, which can cause dampening of the RF fields of the signal. Instability can occur when an RF mode grows (e.g., when more power is put into mode than is dissipated out of the mode).
The SBK, when operating without instability, can have a very high average (or peak) power along with a relatively light weight structure, which can be useful in a variety of scientific, commercial, and military applications. The electron beam in the SBK is flat and can be extended laterally in the shape of “sheet” (hence the name “sheet beam”), thus the electron beam can therefore carry a higher current due to the lower current density. The technology (systems, devices, and methods) described herein provides mechanisms to change the characteristics of the transverse modes and improve the stability of the electron beam of an electron device, such as a SBK.